Grown photonic crystals in semiconductor light emitting devices

ABSTRACT

A photonic crystal is grown within a semiconductor structure, such as a III-nitride structure, which includes a light emitting region disposed between an n-type region and a p-type region. The photonic crystal may be multiple regions of semiconductor material separated by a material having a different refractive index than the semiconductor material. For example, the photonic crystal may be posts of semiconductor material grown in the structure and separated by air gaps or regions of masking material. Growing the photonic crystal, rather than etching a photonic crystal into an already-grown semiconductor layer, avoids damage caused by etching which may reduce efficiency, and provides uninterrupted, planar surfaces on which to form electric contacts.

BACKGROUND

1. Field of Invention

This invention relates to semiconductor light emitting devices includingphotonic crystal structures.

2. Description of Related Art

Light emitting devices such as light emitting diodes (“LEDs”) aretechnologically and economically advantageous solid state light sources.LEDs are capable of reliably providing light with high brightness, hencein the past decades they have come to play a critical role in numerousapplications, including flat-panel displays, traffic lights, and opticalcommunications. An LED includes a forward biased p-n junction. Whendriven by a current, electrons and holes are injected into the junctionregion, where they recombine and release their energy by emittingphotons. Materials systems currently of interest in the manufacture ofhigh-brightness light emitting devices capable of operation across thevisible spectrum include Group III-V semiconductors, particularlybinary, ternary, and quaternary alloys of gallium, aluminum, indium, andnitrogen, also referred to as III-nitride materials.

The quality of an LED can be characterized, for example, by itsextraction efficiency, which measures the ratio of photons extractedfrom the device to photons generated in the light emitting region. Theextraction efficiency is limited, for example, by the emitted photonssuffering multiple total internal reflections at the walls of the highrefractive index semiconductor crystal that forms the p-type, n-type,and light emitting regions of the device. As a result, many of theemitted photons do not escape into free space, leading to poorextraction efficiencies, typically less than 30%.

Various approaches have been proposed to enhance the extractionefficiency of LEDs. The extraction efficiency can be increased, forexample, by enlarging the spatial angle in which the emitted photons canescape by developing suitable geometries, including cubic, cylindrical,pyramidal, and dome like shapes. However, none of these geometries canentirely eliminate losses from total internal reflection.

A further source of loss is the reflection caused by the refractiveindex mismatch between the LED and the surrounding media. While suchlosses could be reduced with an anti-reflection coating, completecancellation of reflection can be achieved only at a specific photonenergy and one angle of incidence.

U.S. Pat. No. 5,955,749, entitled “Light Emitting Device Utilizing aPeriodic Dielectric Structure,” granted to J. Joannopoulos et al.,describes an approach to the problem of enhancing the extractionefficiency. According to U.S. Pat. No. 5,955,749, a photonic crystal iscreated by forming a lattice of holes completely through thesemiconductor layers of the light emitting diode. The lattice of holescreates a medium with a periodically modulated dielectric constant,affecting the way light propagates through the medium. The photons ofthe light emitting diode can be characterized by their spectrum ordispersion relation, describing the relation between the energy and thewavelength of the photons. The relationship may be plotted, yielding aphotonic band diagram consisting of energy bands, or photonic bands,separated by band gaps. Though the photonic band diagram is analogous tothe spectrum of electrons in crystalline lattices as expressed in anelectronic band diagram, the photonic band diagram is unrelated to theelectronic band diagram. When a photonic crystal is formed in an LED itaffects how light propagates in the structure. Therefore if the properlattice spacing is chosen, light that would otherwise have been trappedin the structure by total internal reflection can now escape, increasingthe extraction of the LED. Also, alternative lattices can reduce thephoton mode volume in the LED structure increasing the radiative rate orinternal efficiency of the LED active layer.

U.S. Pat. No. 5,955,749 does not teach how to form a complete,functional light emitting device, and proposes forming photonic crystallight emitting devices in GaAs-based crystals. Needed in the art aredesigns for III-nitride photonic crystal light emitting devices andmethods of making such devices.

SUMMARY

In accordance with embodiments of the invention, a photonic crystal isgrown within a semiconductor structure, such as a III-nitride structure,which includes a light emitting region disposed between an n-type regionand a p-type region. The photonic crystal may be multiple regions ofsemiconductor material separated by a material having a differentrefractive index than the semiconductor material. For example, thephotonic crystal may be posts of semiconductor material grown in thestructure and separated by air gaps or regions of masking material.Growing the photonic crystal, rather than etching a photonic crystalinto an already-grown semiconductor layer, avoids damage caused byetching which may reduce efficiency, and provides uninterrupted, planarsurfaces on which to form electric contacts.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross sectional view of a III-nitride photonic crystal lightemitting device.

FIG. 2 is a cross sectional view of a device with the light emittingregion formed within semiconductor posts that form a photonic crystal.An uninterrupted, planar p-type region is grown over the semiconductorposts.

FIG. 3 is a cross sectional view of a device with the light emittingregion formed within semiconductor posts that form a photonic crystal.The semiconductor posts are bonded to a second semiconductor structureformed on a second growth substrate.

FIG. 4 is a cross sectional view of a device with the light emittingregion formed within semiconductor posts that form a photonic crystal.The semiconductor posts are metal bonded to a host substrate.

FIG. 5 is a cross sectional view of a device with the light emittingregion formed within semiconductor posts that form a photonic crystal,where the semiconductor posts are grown through thick mask layers.

FIGS. 6A-6C illustrate an alternative method of forming the devicesillustrated in FIGS. 2-4.

FIG. 7 is a cross sectional view of a device with an uninterrupted,planar active region and a photonic crystal formed after the activeregion.

FIG. 8 is a cross sectional view of a device with two photonic crystalsseparated by an uninterrupted, planar active region.

FIG. 9 is a cross sectional view of an alternative embodiment of adevice with an uninterrupted, planar active region and a photoniccrystal formed after the active region.

FIG. 10 is a cross sectional view of a device with an uninterrupted,planar active region and a photonic crystal formed before the activeregion.

FIG. 11 is a top view of an example of a photonic crystal lattice ofmaterials of varying index of refraction.

FIG. 12 illustrates the device of FIG. 2 processed into a thin filmdevice.

FIGS. 13 and 14 illustrate two examples of arrangements that form aquasi-crystal.

DETAILED DESCRIPTION

FIG. 1 illustrates a III-nitride photonic crystal LED (PXLED) 100,described in more detail in Publication No. 2003/0141507, “LEDEfficiency Using Photonic Crystal Structure,” filed Jan. 28, 2002 andincorporated herein by reference.

In PXLED 100 of FIG. 1, an n-type region 108 is formed over growthsubstrate 102 which may be, for example, sapphire, SiC, or GaN; anactive region 112 is formed over n-type region 108; and a p-type region116 is formed over active region 112. Each of regions 108, 112, and 116may be a single layer or multiple layers of the same or differentcomposition, thickness, or dopant concentration. A portion of p-typeregion 116 and active region 112 are etched away to expose a portion ofn-type region 108, then a p-contact 120 is formed on p-type region 116and an n-contact 104 is formed on the exposed portion of n-type region108. The device may be flipped over, as illustrated in FIG. 1, andconnected to a mount (not shown) through contacts 104 and 120.

Active region 112 includes a junction region where electrons from n-typeregion 108 combine with holes of p-type region 116 and ideally emitenergy in the form of photons. Active layer 112 may include a quantumwell structure to optimize the generation of photons. Many differentquantum well structures have been described, for example, by G. B.Stringfellow and M. George Craford in “High Brightness Light EmittingDiodes,” published by the Associated Press in 1997. The photonic crystalof PXLED 100 of FIG. 1 is created by forming a periodic structure ofholes 122-i in the LED.

In the device illustrated in FIG. 1, a conventional III-nitridestructure is fabricated with the n-type region formed first on thesubstrate, followed by the active region and the p-type region. Thephotonic crystal device illustrated in FIG. 1 and the devices describedin U.S. Pat. No. 5,955,749 may have several disadvantages. For example,the photonic crystal structure in the device of FIG. 1 may be formed bydry etching into the p-type region to form an array of holes that form aperiodic structure. Dry etching may be reactive ion, inductively coupledplasma, focused ion beam, sputter, electron cyclotron resonance, orchemically assisted ion beam etching. Dry etching of p-type material isproblematic because etching can damage the crystal, causing nitrogenvacancies which create n-type donors. In p-type region 116, the presenceof n-type donors lowers the concentration of holes and, in cases ofsevere damage to the crystal, can change the conductivity type of region116 to n-type. The inventors have discovered that the damage caused bydry etching is not limited to a localized area around the etched region,and may propagate vertically and laterally through the non-etched areasof the crystal, possibly eliminating the p-n junction and rendering thedevice electrically non-operational. The devices described in U.S. Pat.No. 5,955,749 also etch through p-type material, and therefore maysuffer from the same widespread damage observed by the inventors. Also,though modeling has demonstrated that it is desirable to form thephotonic crystal very close to or through the active region as in FIG.1, etching through quantum wells in the active region creates surfacerecombination, potentially lowering the efficiency of the device.

In accordance with embodiments of the invention, a photonic crystal isgrown in a semiconductor light emitting device, rather than etched. FIG.11 is a top view of an example of a photonic crystal. A region 2 isinterrupted by regions 4 of a material of different index of refraction.For example, the photonic crystal of FIG. 11 may be an array ofsemiconductor posts 4 of grown semiconductor material, rather than anarray of holes etched into semiconductor material as described above,surrounded by an air region 2. Alternatively, region 2 may be asemiconductor region grown with holes 4.

The photonic crystal structure can include a periodic variation of thethickness of one of the semiconductor, regions, with alternating maximaand minima. An example is a grating (one-dimensional lattice) or planarlattice of posts of semiconductor material, as described above(two-dimensional lattice). The lattice is characterized by the diameterof the posts, d, the lattice constant a, which measures the distancebetween the centers of nearest neighbor posts, the height of the postsw, and the dielectric constant of the dielectric, disposed around theposts, ∈_(h). Parameters a, d, w, and ∈_(h) influence the density ofstates of the bands, and in particular, the density of states at theband edges of the photonic crystal's spectrum. Parameters a, d, w, and∈_(h) thus influence the radiation pattern emitted by the device, andcan be selected to enhance the extraction efficiency from the device.Alternatively, when the proper photonic crystal parameters are chosen,the radiation pattern of the emitted light can be narrowed, increasingthe radiance of the LED. This is useful in applications where light atonly specific angles is useful. In one embodiment, the photonic crystalparameters are chosen such that greater than 50% of radiation exitingthe device is emitted in an exit cone defined by an angle of 45 degreesto an axis normal to a surface of the device.

The posts can be arranged to form triangular, square, hexagonal,honeycomb, or other well-known two-dimensional lattice types. Multiplelattice types may be included in the same region of the device or beused to create a quasi-crystal, which may provide greater control overlight propagation from the device. FIGS. 13 and 14 illustrate twoexamples of an arrangement of holes that form a quasi-crystal. Asillustrated in FIGS. 13 and 14, a quasi-crystal is a pattern of holeslocated on the vertices of a repeating pattern of squares 131 andtriangles 132. Such a repeating pattern is often referred to as anArchimedean lattice or a penrose tile. The lattice constant a of aquasi-crystal is the length of a side of a triangle or square in therepeating pattern.

In some embodiments, different lattice types are formed in differentregions of the device. For example, one photonic crystal structuredesigned to optimize total radiative power (radiative efficiency) may beformed in one region of the device, and another photonic crystalstructure designed to optimize light extraction (extraction efficiency)may be formed in another region of the device.

The posts often have hexagonal cross sections, though other crosssections are possible. In some embodiments, the lattice spacing a isbetween about 0.1λ and about 10λ, more preferably between about 0.1λ andabout 5λ, more preferably between about 0.1λ and about 3λ, and morepreferably between about 0.1λ and about λ, where λ is the wavelength inthe device of light emitted by the active region. In some embodiments,the lattice constant a is selected to be in or near the bandgap of thephotonic crystal. For example, in a triangular lattice of holes formedin a GaN layer and filled with air, a lattice constant ranging from0.35λ to 0.55λ is in the bandgap favoring extraction at the lower end ofthe range and at the bandedge favoring internal efficiency at the higherend of the range. The lattice constant range 0.35λ to 0.55λ assumes apost radius of 0.36 a. In some embodiments, the posts may have adiameter d between about 0.1 a and about 0.5 a, where a is the latticeconstant. The posts can be surrounded with air or with an optionaldielectric of dielectric constant ∈_(h), often between about 1 and about16. Possible dielectrics include silicon oxides, which may be the masklayers described in the below examples. In some embodiments, the heightof the posts w is at least 0.25λ. The height of the posts is limited bythe preference for forming electrical contacts on planar layers withoutposts, thus the posts cannot extend the entire height of the device. Inembodiments where the light emitting region is a planar layeruninterrupted by the photonic crystal and the photonic crystal islocated above or below the light emitting region, the photonic crystalpreferably extends to within 3λ of the light emitting region.

In some embodiments of the invention, the light emitting region isincluded in the posts of grown semiconductor material that form thephotonic crystal. FIGS. 2-5 and 6A-6C illustrate examples of suchdevices. In the device of FIG. 2, an n-type region 22 is conventionallygrown over any suitable substrate 20 such as, for example, sapphire,SiC, or GaN. N-type region 22 often includes multiple layers ofdifferent thickness, composition, and dopant concentration, such as, forexample, optional preparation layers such as buffer layers or nucleationlayers designed to provide a high quality template on which n-typedevice layers such as an n-type cladding layer and an n-type contactlayer are grown. N-type region 22 may include layers that are notintentionally doped. In embodiments where growth substrate 20 isremoved, n-type region 22 may include layers which facilitate release ofgrowth substrate 20 or facilitate thinning of the epitaxial layers aftergrowth substrate 20 is removed.

Over the planar n-type region 22, a mask layer 24 such as SiO₂ isformed. Mask layer 24 may be a thin layer, for example, with a thicknessless than 200 nm. Openings are formed in mask 24. Posts of semiconductormaterial which will form the photonic crystal are grown in the openings.Posts 26 of n-type material are grown first, followed by posts 28 oflight emitting region material. In the examples described herein, thelight emitting region may be any suitable light emitting regionstructure including, for example, a single thick light emitting layer, asingle thin quantum well, multiple thin quantum wells separated bybarrier layers, and multiple thick light emitting layers separated bybarrier layers. After the light emitting region 28, posts 30 of p-typematerial are grown. The size, height, spacing, and organization of theposts may be selected to form a photonic crystal, as described above.

The semiconductor posts may be formed by, for example, low-pressuremetalorganic chemical vapor deposition as described by Kipshidze et al.,“Controlled growth of GaN nanowires by pulsed metalorganic chemicalvapor deposition,” Applied Physics Letters 86, 033104 (2005) which isincorporated herein by reference. Growth may be carried out at lowpressure, for example 30 Torr, using group III precursors such astrimethylgallium and group V precursors such as ammonia, with N₂ as acarrier gas. Growth is nucleated by islands of a metal catalyst such asnickel formed 2-5 nm thick in the openings in mask 24. The nickelbecomes supersatured with Ga and active nitrogen supplied from the gasphase. Once nucleated, growth of the semiconductor post occurs at thesolid-liquid interface between the semiconductor and the nickel. Growthmay be pulsed to avoid gas phase reactions between the growth gases,such that a pulse of group V precursor is introduced to the reactor fora given time, followed by a delay, followed by a pulse of group IIIprecursor. The resulting posts grow perpendicular to the substratesurface, with constant diameters and smooth sidewalls.

Alternatively, the posts can be grown as described by S. Hoffouz et al.,“Effect of Magnesium and Silicon on the lateral overgrowth of GaNpatterned substrates by Metal Organic Vapor Phase Epitaxy,” MRS InternetJ. Nitride Semicond. Res. 3, 8 (1998) which is incorporated herein byreference. Hoffuz describes patterning the growth substrate surface withmicron sized openings in a growth mask. The mask prevents growth on topof the mask. Growth starts in the mask openings and can proceed in alateral overgrowth mode up and over the mask, or can grow preferentiallyvertical depending on the type and concentration of dopant that isflowed in during growth. For example if columnar growth is desired thenhigh SiH₄ flow is used during growth. If lateral overgrowth is desiredthen low flow of SiH₄ or Cp₂Mg is used.

After p-type posts 30 are grown, the growth conditions are changed suchthat inverted pyramids are formed over the posts, which pyramidseventually connect to form a planar layer 32 over the posts and spaces25 between the posts. If used, the metal catalyst is removed prior togrowth of planar layer 32. Hoffouz et al., incorporated above, describeslateral overgrowth techniques for p-type Mg-doped III-nitride materials.P-type posts 30 and planar p-type region 32 may include multiple layersof different thickness, composition, and dopant concentration, such as,for example, a p-type cladding layer and a p-type contact layer.

After growth of planar p-type region 32, one or more metal layers whichform a p-contact (not shown) are deposited on p-type region 32. Thep-contact may include multiple layers such as an ohmic contact layer, areflective layer, and a guard metal layer. The reflective layer is oftensilver or aluminum. The guard metal may include, for example, nickel,titanium, or tungsten. The guard metal may be chosen to prevent thereflective metal layer from migrating, particularly in the case of asilver reflective layer, and to provide an adhesion layer for a bondinglayer used to bond the semiconductor structure to a host substrate.

The semiconductor structure may then be metal bonded to a host substrateand processed into a thin film device, as illustrated in FIG. 12. One ormore bonding layers 90, typically metal, may serve as compliantmaterials for thermo-compression or eutectic bonding between thesemiconductor structure and host substrate 92. Examples of suitablebonding layer metals include gold and silver. Host substrate 92 providesmechanical support to the semiconductor layers after the growthsubstrate is removed, and provides electrical contact to the p-typeregion through metal layers 90. The host substrate is selected to beelectrically conductive (i.e. less than about 0.1 Ωcm), to be thermallyconductive, to have a coefficient of thermal expansion matched to thatof the semiconductor layers, and to be flat (i.e. with an root meansquare roughness less than about 10 nm) enough to form a strong bond.Suitable materials include, for example, metals such as Cu, Mo, Cu/Mo,and Cu/W; semiconductors with metal contacts 94, such as Si with ohmiccontacts and GaAs with ohmic contacts including, for example, one ormore of Pd, Ge, Ti, Au, Ni, Ag; and ceramics such as compressed diamondand AlN.

The host substrate and semiconductor structure are pressed together atelevated temperature and pressure to form a durable metal bond betweenthe bonding layers. In some embodiments, bonding is done on a waferscale, before a wafer with a semiconductor structure is diced intoindividual devices. Alternatively, bonding is done on a die scale aftera wafer with the semiconductor structure is diced into individualdevices. The temperature and pressure ranges for bonding are limited onthe lower end by the strength of the resulting bond, and on the higherend by the stability of the host substrate and the semiconductorstructure. For example, high temperatures and/or high pressures cancause decomposition of the epitaxial layers in the semiconductorstructure, delamination of the p-contact, failure of diffusion barriers,for example in the p-contact, or outgassing of the component materialsin the semiconductor layers. A suitable temperature range is, forexample, about 200° C. to about 500° C. A suitable pressure range is,for example, about 100 psi to about 300 psi.

After bonding to the host substrate, growth substrate 20 may be removedby a technique appropriate to the growth substrate material. A sapphiregrowth substrate may be removed by, for example, laser melting. Otherremoval techniques include etching and lapping. Once the growthsubstrate is removed, n-type region 22 may be thinned to a desiredthickness or to remove low quality preparation layers adjacent to growthsubstrate 20. An n-contact 96 may then be formed on the exposed surfaceof n-type region 22. Since the p-contact is reflective, light isextracted from the device through the exposed surface of n-type region22.

Alternatively, the device of FIG. 2 may be processed into a flip chip,where growth substrate 20 remains on the device and light is extractedfrom the device through the growth substrate. A portion of p-typeregions 30 and 32 and light emitting region 28 are removed to expose apart of one of n-type regions 22 or 24. An n-contact is formed on theexposed part of one of the n-type regions and a p-contact is formed onthe remaining portion of p-type region 32. The die is flip-chipconnected to a carrier and light is extracted from the device throughsubstrate 20. To further improve light extraction substrate 20 may beremoved by underfilling the die for mechanical support of thesemiconductor structure, then removing the substrate by, for example,laser lift-off, etching, or lapping.

FIG. 3 illustrates another example of a device where the light emittingregion is included in the posts of grown semiconductor material thatform the photonic crystal. N-type region 22, n-type posts 26, lightemitting region posts 28, and p-type posts 30 are grown as describedabove in reference to FIG. 2. Rather than growing a planar p-type region32 over the posts, growth is stopped after growth of the posts and thestructure is bonded to a semiconductor region 40 grown on a secondgrowth substrate 42. Semiconductor region 40 may be, for example, ap-type region.

The top surface of the semiconductor posts and the surface ofsemiconductor region 40 are bonded together under elevated temperatureand pressure. An appropriate temperature for bonding may be, forexample, between 700 and 1200° C.; an appropriate pressure for bondingmay be, for example, between 5 and 1500 psi. The surfaces may be pressedtogether at the above temperature and pressure in an atmosphere of, forexample, N₂ or NH₃ for a specified time period, for example, at least 2minutes, often for at least 30 minutes. Under these conditions, a robustsemiconductor bond is formed between the two surfaces. Such a bond maywithstand the temperatures necessary for further semiconductorprocessing subsequent to bonding, such as growing additionalsemiconductor layers. Other bonding techniques besides semiconductorwafer bonding may be used, such as diffusion soldering bonding. In adiffusion soldering bond, one or more metals such as Zn and Sn aredeposited at the bonded interface and bonded at low temperature. TheZnSn bond is stable at high temperatures, for example temperaturesgreater than 900° C. In another alternative method, the surfaces arebonded using a thin metallic film such as Al as a bonding layer. The Almay alloy into the two semiconductor surfaces, creating a bond that isstable at high temperatures.

After bonding, one of growth substrate 20 and growth substrate 42 may beremoved, exposing a semiconductor surface. Contacts may be more readilyformed on the uninterrupted, planar surface exposed by removal of eithersubstrate than on the tops of p-type posts 30. After growth substrate 42is removed, the resulting device may be processed into a thin filmdevice or a flip chip, both described above in reference to FIG. 2.

The device of FIG. 4 is like the device of FIG. 3 except that thesemiconductor posts are metal bonded 44 to a host substrate 46, asdescribed above in reference to FIG. 2. Growth substrate 20 may beremoved, as described above in reference to FIG. 2.

In each of the devices illustrated in FIGS. 2, 3, and 4, thesemiconductor posts that form the photonic crystal are grown throughopenings in a thin mask layer 24. In the device illustrated in FIG. 5,the semiconductor posts are grown through openings in a thick mask layer48. Mask 48 may be SiO₂ with a thickness greater than 0.25λ. Theopenings in mask 48 may be arranged such that the semiconductor materialgrown in the openings forms an array of posts. Alternatively, mask 48may be formed such that the mask forms the array of posts, and thesemiconductor material fills in the areas between the mask materialposts. The mask confines growth to the area between the masked regions,thus special growth techniques are not necessary, except higher pressureduring growth, for example, greater than 100 Torr may be necessary toforce the precursor gases into the openings in mask 48. Once thesemiconductor posts reach the top of mask 48, p-type region 50 grows inan uninterrupted, planar sheet. Mask 48 may be left in the gaps betweenthe semiconductor posts, as illustrated in FIG. 5, or removed prior togrowth of planar p-type region 50, to create air gaps between thesemiconductor posts.

FIGS. 6A-6C illustrate an alternative method of growing the devicesillustrated in FIGS. 2-4, where the semiconductor posts are grown on atemplate formed by dry etching. As illustrated in FIG. 6A, a planarn-type region 22 is grown over substrate 20. A mask layer 52 is formedover n-type region 22, then a portion of n-type region 22 is etched awayto form n-type posts 54. Mask 52 is removed, and growth is resumed. Thegrowth conditions are selected such that nucleation does not occur inthe areas between posts 54, and growth continues only on posts 54. Forexample, growth at low pressure, for example less than 100 Torr, orgrowth at high n-type dopant precursor flow may confine growth to thetops of posts 54. Light emitting region posts 28 are grown, followed byp-type region posts 30, as described above. The device may be finishedby growing an uninterrupted, planar p-type region as illustrated in FIG.2, bonding the semiconductor posts to a semiconductor region asillustrated in FIG. 3, or metal bonding the semiconductor posts to ahost substrate as illustrated in FIG. 4.

In the embodiments illustrated in FIGS. 2-5 and 6A-6C, light emittingregion 28 is located within the semiconductor posts that form thephotonic crystal. Growth of the semiconductor posts permits the photoniccrystal to be formed at a depth which is difficult to achieve by maskingand etching. In addition, growing the semiconductor posts avoids thedamage and high surface recombination caused by formation of thephotonic crystal by dry etching. Such damage is particularly problematicfor the light emitting region.

In some embodiments of the invention, the light emitting region is aplanar layer uninterrupted by the photonic crystal, and formed beforeand/or after the posts of grown semiconductor material that form thephotonic crystal. FIGS. 7-10 illustrate examples of such devices. In thedevice illustrated in FIG. 7, an n-type region 22 is grown over asubstrate 20, as described above. An uninterrupted, planar lightemitting region 34 is grown over n-type region 22, followed by a thinp-type region 36. For example, as described above, p-type region 36 mayhave a thickness less than 3λ. As the thickness between the photoniccrystal and the light emitting region is reduced, device performance isexpected to improve, thus p-type region 36 is grown as thin as possible.A thin mask layer with openings is then formed over p-type region 36,similar to the mask formed on n-type region 22 as described above inreference to FIG. 2. P-type semiconductor posts, which form the photoniccrystal, are grown through the openings in mask 24. After growth ofp-type posts 38, either a planar p-type region 32 may be grown over theposts, as illustrated in FIG. 7, or a semiconductor region may be bondedto posts 38 as illustrated in FIG. 3, or a host substrate may be metalbonded to posts 38 as illustrated in FIG. 4.

Alternatively, the photonic crystal may be located below the activeregion, in the n-type region, as illustrated in FIG. 10. In the deviceillustrated in FIG. 10, a planar n-type region 22 is grown over growthsubstrate 20. N-type posts 26, which form the photonic crystal, aregrown through openings in mask 24. Uninterrupted, planar n-type region80, light emitting region 82, and p-type region 84 are disposed overn-type posts 26. N-type region 80, light emitting region 82, and p-typeregion 84 may be grown over posts 26, or they may be grown on a separategrowth substrate and bonded to n-type posts 26 as described above inreference to FIG. 3.

The device illustrated in FIG. 8 includes two photonic crystals disposedon opposite sides of a planar light emitting region. A thick mask layer56 with openings is formed over n-type region 22. Mask 56 may be, forexample, at least 0.25λ thick. N-type posts 58, which form the firstphotonic crystal, are grown through the openings in mask 56. Growth ofn-type material continues over mask 56 to form an uninterrupted, planarn-type region 60. A light emitting region 34 is grown over n-type region60, followed by a thin p-type region 36. As described above, n-typeregion 60 and p-type region 36 may each have a thickness less than 3λ. Asecond thick mask layer 62 with openings is formed over p-type region36, then p-type posts 64 are grown through the openings in mask 62,followed by an uninterrupted, planar p-type region 66, which grows overmask 62. In some embodiments, the gaps in the semiconductor in the twophotonic crystals illustrated in FIG. 8 are aligned.

In the device illustrated in FIG. 9, the photonic crystal is formed notfrom posts, as described above, but from pyramids. Pyramids result whenIII-nitride material is grown conventionally through openings in a mask,as opposed to the growth techniques for forming posts described above inreference to FIG. 2. In the device of FIG. 9, an n-type region 22 isformed over growth substrate 20, followed by a light emitting region 34and a thin p-type region 36. A mask layer 70 with openings is depositedover p-type region 36, then p-type material 72 is grown first in theopenings in mask 70, then over the mask to form pyramids 74. Growth isstopped before gaps 76 between pyramids 74 are filled in. Asemiconductor region 40, often p-type and grown on a separate growthsubstrate, is then bonded to pyramids 74. Either growth substrate 20 orthe growth substrate on which semiconductor region 40 is grown (notshown) is removed. As an alternative to semiconductor region 40, a hostsubstrate may be metal bonded to pyramids 74. Also, a layer thatpreserves the gaps 76 between pyramids 74, but ends in a planar,uninterrupted surface at the top, may be grown over pyramids 74.

Alternatively, mask 70 of FIG. 9 may be deposited over n-type region 22,such that material 72 grown in the mask openings and pyramids 74 aren-type. An uninterrupted, planar p-type region, active region, andoptional n-type region may be grown on a separate growth substrate, thenbonded to n-type pyramids 74.

Embodiments of the invention offer several advantages. Damage caused byetching the photonic crystal is avoided, thus devices incorporating agrown photonic crystal may offer improved efficiency over devices withan etched photonic crystal. Also, in embodiments of the invention,planar surfaces on which to make electrical contacts are available onboth the p-type side and the n-type side of the active region. Contactdesign is thus simplified over a device that requires making contact toa layer in which a photonic crystal is formed.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. For example, the device structures in theexamples illustrated above are not limited to the specific growthtechnique described in the example. For example, the device with twophotonic crystals illustrated in FIG. 8 may be grown using thin masklayers, as illustrated in FIG. 2, rather than the thick mask layersillustrated in FIG. 8, or may be grown with the pyramids illustrated inFIG. 9, rather than the posts illustrated in FIG. 8. Therefore, it isnot intended that the scope of the invention be limited to the specificembodiments illustrated and described.

1. A method comprising forming a photonic crystal within a semiconductorstructure, the semiconductor structure comprising a light emittingregion configured to emit light of wavelength λ when forward biased, thelight emitting region being disposed between an n-type region and ap-type region, wherein forming the photonic crystal comprises: forming aplurality of regions of semiconductor material having a first refractiveindex; and forming a plurality of regions of a material having a secondrefractive index, wherein the second refractive index is different fromthe first refractive index; wherein the regions of material having asecond refractive index are disposed between the regions ofsemiconductor material in an array, and the material having a firstrefractive index is grown around the material having a second refractiveindex, and each region of material having a second refractive index islocated less than 5λ from a nearest neighbor region of material having asecond refractive index; and wherein the light emitting region issubstantially planar and the regions of material having a secondrefractive index are confined to a region above or below the lightemitting region.
 2. The method of claim 1 wherein the semiconductorstructure is a III-nitride structure.
 3. The method of claim 1 whereinthe material having a second refractive index is air.
 4. The method ofclaim 1 wherein the material having a second refractive index comprisesone or more of air, an oxide of silicon, a nitride of silicon, anoxy-nitride of silicon, and a dielectric.
 5. The method of claim 1wherein growing a photonic crystal comprises: forming on a semiconductorsurface a mask having a plurality of openings; and growing asemiconductor material through the openings.
 6. The method of claim 5wherein the semiconductor material grown through the openings comprisesa plurality of semiconductor posts.
 7. The method of claim 5 wherein thesemiconductor material has a thickness greater than a thickness of themask.
 8. The method of claim 5 wherein the semiconductor material has athickness substantially the same as a thickness of the mask.
 9. Themethod of claim 6 further comprising providing a metal catalyst in theplurality of openings prior to growing the plurality of posts ofsemiconductor material.
 10. The method of claim 9 wherein the metalcatalyst is nickel.
 11. The method of claim 6 further comprising growinga semiconductor layer having a substantially planar, uninterrupted topsurface over the plurality of posts.
 12. The method of claim 6 whereinthe semiconductor structure is a first semiconductor structure, themethod further comprising bonding a second semiconductor structure tothe plurality of posts.
 13. The method of claim 6 further comprisingbonding a host substrate to the posts by a metal bonding layer disposedbetween the host substrate and the plurality of posts.
 14. The method ofclaim 5 further comprising prior to growing a plurality of posts,etching away a portion of the semiconductor surface to define a photoniccrystal.
 15. The method of claim 5 wherein the photonic crystal isformed in the n-type region.
 16. The method of claim 5 wherein thephotonic crystal is formed in the p-type region.
 17. The method of claim5 wherein the photonic crystal is a first photonic crystal formed in then-type region, the method further comprising forming a second photoniccrystal in the p-type region.
 18. The method of claim 5 wherein at leastone of the plurality of posts has a substantially constant diameter,substantially straight side walls, and grows perpendicular to a surfaceof a growth substrate.
 19. The method of claim 1 wherein the regions ofmaterial having a second refractive index have a thickness of at least¼λ.
 20. The method of claim 1 wherein the regions of material having asecond refractive index are located within 3λ of the light emittingregion.
 21. The method of claim 1 wherein the plurality of regions ofsemiconductor material and the plurality of regions of material having asecond refractive index are arranged in a lattice having a latticeconstant between 0.1λ and 3λ.
 22. The method of claim 1 wherein theplurality of regions of semiconductor material and the plurality ofregions of material having a second refractive index are arranged in alattice having a lattice constant between 0.1λ and λ.
 23. The method ofclaim 1 wherein: the material having a second refractive index is air;and the plurality of regions of semiconductor material and the pluralityof regions of material having a second refractive index are arranged ina lattice having a lattice constant between from 0.35λ and 0.55λ. 24.The method of claim 1 wherein the plurality of regions of semiconductormaterial and the plurality of regions of material having a secondrefractive index are arranged in a linear grating.
 25. The method ofclaim 1 wherein the plurality of regions of semiconductor material andthe plurality of regions of material having a second refractive indexare arranged in a lattice, at least a portion of the lattice comprisingone of a triangular, honeycomb, Archimedean, and quasi-crystal lattice.26. The method of claim 25 wherein a radius of one of the regions ofsemiconductor material is 0.36 multiplied by the lattice constant. 27.The method of claim 1 further comprising: etching away a portion of thep-type region and the light emitting region to expose a portion of then-type region; forming electrical contacts on a remaining portion of thep-type region and the exposed portion of the n-type region; andattaching the electrical contacts to a carrier.
 28. The method of claim27 wherein the semiconductor is grown on a growth substrate, the methodfurther comprising: filling a gap between the carrier and thesemiconductor structure; and removing the growth substrate.
 29. Themethod of claim 1 wherein the semiconductor structure is grown on agrowth substrate, the method further comprising: attaching a surface ofthe semiconductor structure to a carrier; and removing the growthsubstrate.
 30. The method of claim 1 wherein the light emitting layerhas a lateral extent larger than a lateral extent of one of the regionsof semiconductor material having a first refractive index.
 31. Themethod of claim 1 wherein the photonic crystal is disposed between firstand second semiconductor layers, wherein each of the first semiconductorlayer and the second semiconductor layer is a planar semiconductor layerthat is substantially uninterrupted by the photonic crystal.
 32. Themethod of claim 31 wherein the first semiconductor layer is an n-typelayer and the second semiconductor layer is a ptype layer.
 33. Themethod of claim 1 further comprising forming a first electrical contacton the n-type region and a second electrical contact on the p-typeregion, wherein the first and second electrical contacts are formed onplanar semiconductor layers that are uninterrupted by the photoniccrystal.
 34. The method of claim 1 wherein a top surface and a bottomsurface of the semiconductor structure are uninterrupted by the photoniccrystal.
 35. A method comprising growing a photonic crystal within asemiconductor structure, the semiconductor structure comprising a lightemitting region configured to emit light of wavelength λ when forwardbiased, the light emitting region being disposed between an n-typeregion and a p-type region, wherein growing the photonic crystalcomprises: growing a plurality of regions of semiconductor materialhaving a first refractive index; and growing a plurality of regions of amaterial having a second refractive index, wherein the second refractiveindex is different from the first refractive index; wherein the regionsof material having a second refractive index are disposed between theregions of semiconductor material in an array, and each region ofmaterial having a second refractive index is located less than 5λ from anearest neighbor region of material having a second refractive index;and wherein the light emitting region is substantially planar and theregions of material having a second refractive index are confined to aregion above or below the light emitting region.